MOCVD is a method for depositing thin metal or metal compound films on a silicon or other substrate. (In the present disclosure "metal" includes all of the elements of Groups 2B, 2A, 3A, 4A, 5A, and 6A of the Periodic Table except Carbon, Nitrogen, Oxygen, and Sulfur.) The deposited films can be sources of doping impurities which are driven into the substrate, or the films themselves can have different electrical or optical properties than the substrate. These films are used to make laser diodes, solar cells, photocathodes, field effect transistors and other discrete devices, in fiber optic communications, microwave communications, digital audio disc systems, and other advanced optoelectronic technologies. The properties of the film depend on the deposition conditions and the chemical identity of the deposited film.
A special advantage of MOCVD is that organometallic compounds can be found which have much higher vapor pressures at moderate temperatures than the corresponding metals, and which decompose to release the corresponding metals or form compounds thereof at the 550 to 700 degrees Celsius deposition temperatures which should not be exceeded during fabrication.
Typical apparatus currently in use for MOCVD comprises a bubbler which contains a supply of the organometallic compound chosen for a particular process, a reactor or deposition chamber which contains the substrate on which a film is to be deposited, a source of a carrier gas which is inert to the organometallic compound in the bubbler and either inert or reactive to the compound in the deposition chamber, and optionally sources of other reactants or dopants supplied to the reaction chamber. The bubbler and contents are maintained at a constant and relatively low temperature which typically is above the melting point of the organometallic compound but far below its decomposition temperature. The deposition chamber is typically maintained at a much higher temperature, such as about 550 to 700 degrees Celsius, at which the organometallic compound readily decomposes to release its constituent metal. To operate the MOCVD apparatus, the carrier gas is introduced into the bubbler under the surface of the organometallic compound. Rising bubbles of the carrier gas provide a large, constant contact surface and thus uniformly vaporize the organometallic compound. The carrier gas and vapor collected in the headspace of the bubbler are continuously directed to the deposition chamber.
While it is possible to vaporize a sublimable solid organometallic compound in a bubbler, it is difficult to control its rate of vaporization. The surface area of a solid exposed to the carrier gas changes as vaporization proceeds. In contrast, a liquid contained in a bubbler with substantially vertical walls presents the same surface area of a solid to the carrier gas so long as the flow and bubble size of the carrier gas remains steady. Thus, organometallic compounds for MOCVD desirably are liquids at or slightly above room temperature (from about -20.degree. C. to about 40.degree. C.). Such compounds also should have a vapor pressure of at least about 1.0 torrs at the bubbler temperature, boil and decompose at temperatures substantially exceeding the bubbler temperature, and decompose readily at the temperature encountered in the deposition chamber.